I would highly recommend the GTX 460 over the HD5830. It is a better card and if you are concerned about length it is much shorter as well.
Dont forget the OC'ing, temps, and power consumption! The GTX 460 is even a little quieter. Until the 5830 is about $180 or below, the GTX 460 eats it alive. Heres a comparison:http://www.anandtech.com/bench/Product/171?vs=180
Gotta love the new anandtech ssd and gpu bench